At present, the study of the processes of mantle metasomatism is one of the most important areas of mantle petrology. According to modern concepts, sulfur-bearing fluids, sulfide melts, and components of the C–O–H–S fluid, such as CH4, H2S, and CS2, can be considered as potential reducing metasomatic agents [1]. The main sources of reduced sulfur-enriched melts and fluids in the mantle are processes occurring either in subduction zones, or at the boundary between the core and mantle, as well as under the influence of plume matter on mantle rocks [2, 3]. At the same time, it is assumed that the genesis of part of mantle sulfides can be associated with sulfidation reactions that occur when sulfur-containing fluids or melts affect on Mg,Fe,Ni-silicates of mantle ultrabasic rocks. Pioneer experimental studies on modeling the sulfidation reactions of olivine, pyroxenes and garnet were carried out in the 1960s-1980s at high temperatures and atmospheric pressure [4, 5], under the conditions of melting of these silicates. Previously, we studied the processes of interaction of natural Fe,Ni-bearing olivine with sulfur melt under P,T-parameters of the lithospheric mantle (sulfur concentration in the system was of 10 mol %) [6], and showed the fundamental possibility of the formation of mantle sulfides or sulfide melts as a result of transition metals extraction from silicate minerals into sulfur melt. This work is a logical continuation of the experimental modeling of Fe,Ni-olivine sulfidation reactions in the lithospheric mantle, and is aimed at studying the processes of interaction of sulfur-bearing metasomatic agents with mantle silicates at significantly lower sulfur concentrations (from 0.1 to 6 mol %) than in previous studies.

The experiments were carried out using a multi-anvil high-pressure apparatus of a “split-sphere” type (BARS) [7] at a pressure of 6.3 GPa, temperatures of 1050 and 1450°С, and durations of 60 and 40 hours, respectively. Olivine from granular spinel lherzolite xenolith (Udachnaya kimberlite pipe, Yakutia) and elemental sulfur (99.99% purity) were used as starting reagents. The composition of the initial olivine corresponded to Mg1.8Fe0.19Ni0.01SiO4, with FeO content of 9.28 wt % and NiO of 0.51 wt %. The sulfur concentration in the system was variable and amounted to 0.1, 2.0, and 6.0 mol %. Olivine and sulfur were preliminarily ground to a powder with a grain sizes of 10–20 μm, then the reagents were thoroughly mixed with each other and grounded again. Taking into account the previous experience of working with sulfides and sulfur at high P,T-parameters [6, 8, 9], graphite was chosen as the optimal capsule material. The study of phase relationships and compositions of the obtained phases was carried out using the methods of scanning electron microscopy and energy dispersive spectroscopy (TESCAN MIRA 3 LMU and JSM-6510LV), as well as microprobe analysis (Jeol JXA-8100). The structural features of the obtained mineral phases were studied using Raman spectroscopy (a Horiba J.Y. LabRAM HR800 spectrometer equipped with an Olympus BX41 microscope). Analytical studies were carried out at the IGM SB RAS and the Center for Collective Use of Multielement and Isotope Studies of the SB RAS; the details of the applied analytical procedures are presented in [10].

The results of experimental studies, as well as the composition of the obtained silicate and sulfide phases are shown in Tables 1–3. It should be noted that the melting temperature of sulfur at 6.3 GPa is ~870°C [12]. It has been established that the interaction of olivine with a sulfur melt at 1050°C (Xs = 0.1 mol %) leads to the recrystallization of olivine, as well as the formation of single crystals of newly formed orthopyroxene (Mg1.88Fe0.19Si2.01O6, Table 2) and sulfides—Ni-pyrrhotite Fe0.80Ni0.11S and heazlewoodite Ni2.9Fe0.1S2 (Fig. 1a, Table 3). At the same time, the composition of olivine slightly differs from the initial one (Fig. 2a), and its crystal sizes increase by 2–4 times. With an increase in Xs to 2.0 mol %, an aggregate of recrystallized olivine is formed (Fig. 1b), which is characterized by sulfide inclusions and a zonal structure of crystals—with an FeO-rich center (Mg1.80Fe0.19Ni0.01SiO4) and MgO-enriched periphery (Mg1.87Fe0.14Ni0.01SiO4) (Fig. 2b). Numerous small crystals of newly formed pyrite Fe0.96Ni0.03S2 and orthopyroxene Mg1.92Fe0.07Si2O6 were found in the interstices of the resulting polycrystalline aggregate. At the highest sulfur concentration (6.0 mol %), an association of recrystallized zoned olivine (center—Mg1.80Fe0.20Ni0.01SiO4, periphery—Mg1.83Fe0.13SiO4) was obtained (Fig. 2c), as well as newly formed orthopyroxene Mg1.85Fe0.14Si2O6 and pyrite Fe0.96Ni0.03S2 coexisting with the sulfur melt with dissolved components (Fig. 1c, Tables 2, 3). It should be emphasized that for the peripheral parts of zoned olivine crystals, a decrease in iron content was found with an increase in sulfur concentrations in the system (and a constant temperature of 1050°C) (Figs. 2a–2c). In general, the FeO content in the peripheral zones of olivine crystals decreases to 9 wt % (Xs = 0.1 mol %), 6.5 wt % (Xs = 2.0 mol %) and 4 wt (Xs = 6.0 mol %), relative to the initial 9.3 wt % (Figs. 2a–2c).

Table 1. Parameters and the results of experiments in the olivine-sulfur system (6.3 GPa)
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Table 2. Compositions of silicates formed in the olivine-sulfur system
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Table 3. Compositions of sulfide phases formed in the olivine-sulfur system
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Fig. 1.
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SEM micrographs (a–c, f) and element distribution maps (d–e) of polished sections of samples obtained in the olivine-sulfur system: a—Ni,Fe-sulfide crystals in the interstices of a polycrystalline olivine aggregate (1050°C, Xs—0.1 mol %); b—polycrystalline aggregate of recrystallized olivine with newly formed sulfides and orthopyroxene in interstices (1050°C, Xs—2 mol %); c—polycrystalline aggregate of olivine, orthopyroxene and pyrite with quenched predominantly sulfur melt (1050°C, Xs–6 mol %); d—inclusions of Ni-pyrrhotite in olivine, as well as orthopyroxene crystals in interstices (1450°C, Xs—0.1 mol %); e—olivine crystals with sulfide inclusions, in contact with sulfide melt (1450°C, Xs—2 mol %); f—pool of the quenched predominantly sulfur melt with a sulfide component in a polycrystalline aggregate of olivine and orthopyroxene (1050°C, Xs—6 mol %); Ol—olivine, Opx—orthopyroxene, Py—pyrite, Po—pyrrhotite, Hs—heazlewoodite, L—predominantly sulfur melt, L1—sulfide melt, L2—predominantly sulfur melt with sulfide component.

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Fig. 2.
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Olivine composition profiles (FeO content) after experiments at variable sulfur concentrations and temperatures of 1050°C (a–c) and 1450°C (d). Each colored line represents the profile of one olivine crystal, recorded using the energy-dispersive spectroscopy method (the step between the analyzed points is 3–5 µm).

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As a result of the reconstruction of the interaction processes, it was found that even at a relatively low temperature (1050°C) and a minimal sulfur content in the system of 0.1 mol %, olivine sulfidation occurs, including its partial recrystallization, accompanied by the extraction of Fe and Ni from olivine into the sulfur melt, as well as crystallization of newly formed Ni,Fe-sulfides and orthopyroxene. At the same time, a characteristic (indicative) feature of these processes is the formation of an association of olivine (slightly changed in composition) with high-nickel sulfides. It has been established that at higher sulfur concentrations (2–6 mol %), in addition to the above processes, as a result of olivine recrystallization, reverse zoning occurs in its crystals (a sharp decrease in NiO and FeO concentrations in the peripheral zones) (Fig. 2b,c) and the formation of sulfide inclusions in these crystals as well.

At a higher temperature of 1450°C and a minimal sulfur concentration (0.1 mol %), olivine recrystallization was established in the system (crystal sizes up to 400 µm, relative to the initial 20 µm), as well as the formation of a small amount of sulfide melt (Fe : Ni : S = 0.6 : 0.4 : 1, atomic), and numerous crystals of orthopyroxene Mg1.81Fe0.17Si2O6 (Figs. 1d, 2d, Tables 2, 3). It should be noted that at a pressure of 6.3 GPa and ~1300°C, pyrite undergoes incongruent melting, with the formation of a sulfur melt and pyrrhotite [13]. At Xs of 2.0 mol %, in parallel with the olivine recrystallization, a sulfide melt (Fe : Ni : S = 0.75 : 0.03 : 1, atomic) is formed and the newly formed orthopyroxene Mg1.94Fe0.05Si2O6 crystallizes (Fig. 1e, Tables 2, 3). Numerous inclusions of sulfide melt are observed in olivine crystals. The size of olivine crystals varies from 30 to 200 µm, while, in contrast to relatively low-temperature experiments, olivine is not characterized by a zonal structure (Fig. 2d), and a sharp decrease in FeO concentrations to 3 wt % is observed in its composition. At the highest sulfur concentration (6.0 mol %), an assemblage of recrystallized olivine—practically iron-free and nickel-free forsterite (Figs. 1f, 2d) with sulfide inclusions, as well as newly formed orthopyroxene and sulfur melt enriched in sulfide component (Fe : Ni : S = 0.14 : 0.01 : 1 atomic) (Table 3) was obtained.

Thus, it has been established that at a relatively high temperature (1450°С), olivine sulfidation includes a number of phase formation processes, the intensity of which depends on the sulfur concentration in the system. During these processes, olivine recrystallization occurs in the sulfur melt, accompanied by (1) partial (Xs ≤ 2 mol %) or almost complete (Xs = 6 mol %) extraction of transition metals (Fe and Ni) from olivine into the melt, (2) the formation of a sulfide component in the melt, as well as (3) the formation of sulfide inclusions in olivine. Depending on the sulfur concentration in the system, the content of the sulfide component and the proportions of Fe/Ni in the resulting melt vary significantly. In particular, at Xs of 0.1 mol %, a high nickel sulfide melt (Me/S = 1, Fe/Ni = 3/2) was obtained; at Xs of 2 mol %, a sulfide melt with elevated sulfur concentrations (Me/S = 0.8, Fe/Ni = 25/1), and at Xs of 6 mol %—predominantly sulfur melt with a dissolved sulfide component (Me/S = 0.15, Fe/Ni = 14/1). The extraction of iron and nickel into the sulfur melt during olivine recrystallization leads to a sharp decrease in the FeO and NiO contents in olivine, down to 3 wt % FeO at Xs ≤ 2 mol % and up to 0.4 wt % at Xs = 6 mol % (at initial concentrations of FeO ~ 9.3 wt %) (Figs. 2d, 3b). The excess of SiO2 relative to MgO, arising in this process, leads to the crystallization of orthopyroxene in association with olivine, as it was assumed in previous studies [5, 12], and the amount of newly formed orthopyroxene is directly proportional to the sulfur concentration in the system.

Fig. 3.
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Graphs of the dependence of the compositions of silicate phases on the sulfur concentration in the system. The compositions of olivine and orthopyroxene obtained in experiments in the olivine-sulfur system with Xs = 10 mol % are taken from the study by Bataleva et al. (2016) [6].

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The obtained experimental results on the assessment of the effect of sulfur concentration on olivine sulfidation under the conditions of the lithospheric mantle made it possible to reveal a number of patterns of phase formation depending on temperature and Xs. It has been established that as a key indicator of the interaction of Fe,Ni-olivine with trace amounts of a sulfur-bearing metasomatic agent (Xs = 0.1 mol %), one can consider the association of high-nickel sulfides (heazlewoodite or Ni-pyrrhotite) with olivine and orthopyroxene, which are very similar in composition to unaltered silicates of mantle peridotites. It has been experimentally demonstrated that olivine crystals with sulfide inclusions and reverse zoning (iron-rich center and magnesian-rich periphery), which are traditionally considered as indicators of sulfidation processes in nature [14, 15], form only at a relatively low temperature and Xs ≥ 2 mol %. It has been established that the main features of the most intense sulfidation processes occurring at elevated temperatures and high concentrations of sulfur-bearing agents (Xs ≥ 6 mol %) is the formation of homogeneous crystals of low-iron, low-nickel olivine with inclusions of Fe,Ni-sulfides, in association with orthopyroxene.

Thus, the ephemeral nature of sulfur in mantle fluids is well illustrated by experiments with low concentrations of S (~0.1 mol %), in which it was demonstrated that as a result of mantle metasomatism with the participation of sulfur-bearing agents, no sulfur remains in the altered rock, since all of it is spent on the formation of sulfides. The data obtained in this experimental study indicate that the reducing sulfur-bearing metasomatic agents, even in minimal concentrations, are capable of dissolving and transporting components of mantle silicate phases, as well as playing a significant role in sulfide ore formation with the participation of sulfur-enriched mantle fluids.